Radical attack and mineralization mechanisms on electrochemical oxidation of p-substituted phenols at boron-doped diamond anodes

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Degradation of phenols with different substituent groups (including –OCH3, –CHO, –NHCOCH3, –NO2, and −Cl) at boron-doped diamond (BDD) anodes has been studied previously based on the removal efficiency and •OH detection. Innovatively, formations of CO2 gas and various inorganic ions were examined to probe the mineralization process combined with quantitative structure-activity relationship (QSAR) analysis. As results, all phenols were efficiently degraded within 8 h with high COD removal efficiency. Three primary intermediates (hydroquinone, 1,4-benzoquinone and catechol) were identified during electrochemical oxidation and degradation pathway was proposed. More importantly, CO2 transformation efficiency ranked as: no N or Cl contained phenols (p-CHO, p-OCH3 and Ph) > N-contained phenols (p-NHCOCH3 and p-NO2) > Cl-contained phenols (p-Cl and o,p-Cl). Carbon mass balance study suggested formation of inorganic carbon (H2CO3, CO32− and HCO3−) and CO2 after organic carbon elimination. Inorganic nitrogen species (NH4+, NO3− and NO2−) and chlorine species (Cl−, ClO3− and ClO4−) were also formed after N- and Cl-contained phenols mineralization, while no volatile nitrogen species were detected. The phenols with electron-withdrawing substituents were easier to be oxidized than those with electron-donating substituents. QSAR analysis indicated that the reaction rate constant (k1) for phenols degradation was highly related to Hammett constant (∑σo,m,p) and energy gap (ELUMO - EHOMO) of the compound (R2 = 0.908), which were key parameters on evaluating the effect of structural moieties on electronic character and the chemical stability upon radical attack for a specific compound. This study presents clear evidence on mineralization mechanisms of phenols degradation at BDD anodes.